1. Field of the Invention
This invention relates to a connecting device for pneumatic and/or hydraulic circuits to be used in the separation technique, especially in capillary column chromatography for systems of partition and commutation of the flow of fluid carrying the sample to be analyzed, wherein chemically inert gaskets and fittings are used.
In the case of gas chromatographic separations in a separation column or in several separation columns coupled in series or in parallel, said columns must be connected between each other and to one or more injectors and detectors, and even to supplementary gas pipings, i.e. pipings for make-up gas or pilot gas, to allow for a time regulated inversion of the gas flow under test from a separation column to another one, according to a known "Deans's commutation technique". In this case the relevant fittings must behave in a chemically inert way with respect to the fluid flows and must ensure a perfect seal without considerably departing from the internal diameter of the separation capilliary column, so as to ensure an almost complete elimination of dead volumes. In this case, "dead volume" means the space not necessary for conveying fluid, in the parts of which said fluid flows in and out, and where the presence of the fluid or gas under test is prolonged (peak broadening), and wherein parts of the sample can be retained and fractions of already separated gas sample are mixed again. Dead volumes, for instance, appear in the points where the diameter of the gas piping increases, where bag extensions are formed in these pipings, where blind tubings are connected, and so on. In operation with one or more separation columns, the pneumatic circuits, which are differentiated from each other according to specific gas chromatographic functions, must be such as to be connected and optimized in a simple way, so that, in particular, also the insertion or substitution of separation glass capillary columns, which are delicate and expensive, can be performed in a way as to exclude damaging or complex operations.
It is known that in the most simple case of a gas chromatographic analysis, a sample vaporized in the injector or introduced therein already at the gaseous state, is conveyed by the carrier gas flow through a separation column, at the end of which a detector is provided. For this process, separation capillary columns are preferable to separation packed columns, due to their higher separation power.
Besides this simpler pneumatic circuit, i.e. injector.fwdarw.separation column.fwdarw.detector, several other pneumatic circuits are known, which can involve the use of one to two injectors, one to three separation columns, one to three detectors; in this case, for example, single separation columns can be washed in countercurrent while analysis is going on.
2. Description of the Prior Art
Pneumatic circuits of said type have been described for instance by D. R. Deans, Chromatographia 1 (1968), 18-22 (U.K. Pat. No. 1 236 937); W. Bertsch, HRC & CC 1 (1978), 85-91; 187-198; 289-300; G. Schomburg, "Gaschromatographie in Kapillaarsaulen", Max Planck Inst. fur Kohlenforschung 40-42, 86 (1979); R. E. Kaiser, "Vorsaule in der Gaschromatographie", Selbstverlag Institut fur Chromatographie; R. E. Kaiser "Trenncassetten zur Gas-Chromatographie", Chromatographia Vol. 7 (1974). A common peculiarity of this technique is the fact that several separation capillary columns are connected to injectors, detectors and to the necessary supplementary pipings. No inert fittings have been known up to now, by means of which said connections could be performed in such a way that diversified pneumatic circuits could be connected and disconnected according to analytical needs.
As a matter of fact, the presently known technique comprises welded fittings, and therefore, not detachable ones, or fittings having at least a metal section, and therefore, not inert with respect to all possible fluids. This known technique also includes a commutation system, especially for gas chromatographic columns, according to German patent application No. 2,655,387 to Siemens A. G. According to said system, the commutation of the gas flow from a pre-column to an auxiliary detector or to a main separation column, is carried-out by a pilot gas by means of a special fitting essentially consisting of a Pt/Ir capillary, which is positioned in another tube together with a coaxial gasket, said tube being provided, at its two ends which are insulated between each other by said gasket, with three fittings for the connection of separation capillary columns and other necessary tubings. By a suitable control of the pilot gas flow, dead volumes, which are unavoidable in such configurations, are made inefficient. Said system allows washing of the precolumn in countercurrent as well as connecting packed separation columns to the described double-tube central fitting, but it has limits due to its catalytic activity and to the specific pneumatic functions described in said patent.